Seismic activity in December 1993 was characterized by continuing swarms of high-frequency events from the same region 3.5 km NNE that was active in late November. In total 200 high-frequency events were registered during December. Eight earthquakes were felt in the city of Pasto and in towns closer to the volcano during December; 10 events were felt in November. On the afternoon of 1 December, the largest event (M 3.3) took place with a focal depth of 3-9 km. From 4 to 30 December there were no reports of felt seismicity. On the evening of 30 December a M 2.7 event was felt in Pasto, followed by a swarm of 10 high-frequency events (M 1-1.5), at a depth of 2.4-7.0 km. Very few of the high-frequency events registered in December were located around the active crater. Six of the earthquakes, though, were fracture events located very close to the main crater at a depth of 2.3-2.8 km.

On 3 December, 63 butterfly-type events were registered. The number of events/day then decreased to 4 by 9 December, and remained low until 22 December. The daily number of events then increased through 28 December, reaching 130 events/day before decreasing again through the end of the month. On 27 December the only long-period screw-type event of the month was registered; background tremor was variable, with small changes in amplitude and frequency.

Electronic tiltmeters remained stable; variations during November and December corresponded to voltage changes when the battery runs low. SO₂ output, measured by COSPEC, remained at the low-levels shown during most of 1993.

Geological Summary. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: INGEOMINAS, Pasto.